1. Technical Field
The present invention relates to bipolar separators for fuel cells and, in particular, bipolar separators with improved flow properties.
2. Description of the Related Art
Electrochemical fuel cells convert reactants to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte interposed between two electrodes, namely a cathode and an anode, to form an electrode assembly, which is typically interposed between two electrically conductive flow field plates or separator plates (separators) made of carbonaceous, graphitic, and/or metallic materials. These separators act as current collectors, provide support for the electrodes, and provide passages for the reactants and products. Such separators may comprise channels to direct the flow of reactants to the anode and the cathode, and to remove excess reactants and their reaction products, such as water formed during fuel cell operation.
Fuel cells may employ bipolar separators having an anode flow field on one surface, and a cathode flow field on the opposing surface. Alternatively, a bipolar separator may be employed having an anode flow field plate with an anode flow field on its active surface, and a cathode flow field plate with a cathode flow field on its active surface, joined together around their peripheral edges to form a coolant flow field between their non-active surfaces. In such cases, the bipolar separator is sealed so the coolant does not leak from the fuel cell.
Typically, a number of fuel cells are electrically coupled in series to form a fuel cell stack. The fuel cell stack may contain supply and exhaust manifolds for directing the flow of reactants to/from the fuel cell stack. In some cases, the anode and cathode plates have header regions (“headers”) between the manifold openings and the respective flow fields. The primary function of such headers is to evenly distribute flow to or from the manifold openings and the respective flow fields. This is accomplished by providing paths for the fluid to travel having various distances (as determined by the width of the flow field) with minimum differences in flow resistance between the various paths.
Existing header configurations generally cause unacceptably high gas velocities where the gas exits the manifold openings and enters the headers, which results in uneven flow (i.e., uneven “sharing”) between the flow channels in the flow field. For example, the flow channels that are closer to the manifold opening will receive reactant at a higher pressure than the flow channels that are farther from the manifold opening, thus resulting in different channel-to-channel flow distributions. Furthermore, manufacturing defects may create slightly different flow channel geometries for each of the flow channels (e.g., different cross-sectional areas), resulting in different flow resistances and, thus, different channel-to-channel flow distributions.
As a result, there remains a need to develop bipolar separators with improved fluid distribution properties. The present invention addresses this issue and provides further related advantages.